Physical Description

Achatinella mustelina from different locations vary in size, shape, and color, but no specific patterns in these differences have been described. In general, adults range 19 – 24 mm in length, with an average length of 21.4 mm. The shells have a shiny finish and are usually brown with light bands that circle the suture convex, or they are white with transverse black or brown lines. Shells of A. mustelina can be either dextral or sinistral and consist of five to seven convex whirls. The shell is relatively high-spiraled and oblong to ovate with an aperture that is also oblong and ovate. The columella of A. mustelina is short, stout, and slightly twisted, and it has a well-developed spiral lamella, or ridge. A callus on the columella closes the umbilicus. The lip of the organism does not have any ridges, ribs, or folds. ("Recovery Plan for the O’ahu Tree Snails of the Genus Achatinella", 1992; Aubry, et al., 2005; Baily, 1943; Hadfield, et al., 1993; Killian, 2007; Schilthuizen and Davison, 2005; Welch, 1938)

Development

Achatinella mustelina develops from an intrauterine embryo, and its growth in utero is thought to be logarithmic. Young are born live and are approximately 4.5 mm in length. The snail grows at a relatively slow rate of 2 mm per year, and over the course of four to five years, it will only grow to be five times its birth length. Growth in A. mustelina is determinate and individuals reach their maximum size before becoming reproductively mature. A unique characteristic of development in A. mustelina is that individuals of different sizes grow at the same rate. (Hadfield and Mountain, 1980; Hadfield, et al., 1993)

Reproduction

Achatinella mustelina has a lengthy gestation period and gives birth to large, live young that mature late and have a low fecundity. Although reproductive maturity occurs after the maximum size is reached, reproductive maturity may be dependent on age, not size. Reported age at maturity varies, ranging from 3 to 6.9 years. Similarly, reports of fecundity vary, but rates as low as 0.4 offspring/adult/year have been observed, as well as rates as high as 7 offspring/adult/year.

Little information about reproductive behavior in Achatinella mustelina is known, but this species is hermaphroditic and thought to breed year-round. Some species of the genus Partulina, sister to Achatinellidae, are thought to self-fertilize, so A. mustelina may self-fertilize as well. Partulina redfieldii can reproduce even in long-term absence of mates. The mechanism of this form of reproduction is not clear, however. Possibilities include parthenogenesis and long-term sperm storage, in addition to self-fertilization.

Although the literature does not include specific information about parental investment in Achatinella mustelina, in general, terrestrial gastropods do not provide parental care after birth. The most common form of parental investment for terrestrial gastropods comes in the form of calcium carbonate and other nutrients given to the developing egg. (Baur, 1994)

Parental Investment

pre-fertilization

provisioning

Lifespan/Longevity

Achatinella mustelina has a relatively long lifespan compared to other terrestrial gastropods. Longevity is estimated to be 10 years, but individuals may live up to 15 or 20 years. (Hadfield, et al., 1993; Killian, 2007)

Typical lifespanStatus: wild

20 (high) years

Average lifespanStatus: wild

10 years

Behavior

The Oahu tree snail is a terrestrial snail that is primarily found in native Hawaiian trees, such as Osmanthus sandwicensis, Gouldia sp., Metrosideros polymorpha, and large bushes. Generally, this snail attaches to leaves at the tops of native trees and shrubs, and is nocturnal. Individuals of A. mustelina enter a stage of inactivity during the numerous dry periods of its environment, curtailing feeding and growth. Individuals are not highly motile, resulting in many small, relatively isolated populations of A. mustelina. (Hadfield, et al., 1993; Holland and Hadfield, 2007; Killian, 2007)

Home Range

The home range of A. mustelina is often limited to a single tree and the nearby surrounding shrubbery. Individuals rarely move between trees.

Communication and Perception

Achatinella mustelina, like most terrestrial gastropods, communicates through both touch and chemical signaling, using the lower set of head tentacles. Individuals transmit pheromone signals both through direct contact and through the mucous trail left during locomotion. This includes warning of predator presence.

Achatinella mustelina has simple eyes for visual perception, albeit limited to larger objects. However, vision is not a primarily means of searching or foraging, given the snail’s nocturnal habits. (Chelazzi, 1990; Pakarinen, 1991)

Food Habits

Achatinella mustelina feeds primarily at night. This species is a mycophage, grazing upon epiphytic fungi growing on the bark or leaves of native plant species, such as Osmanthus sandwicensis, Gouldia sp. or Metrosideros polymorpha. Few imported species of plant are suitable for the fungi eaten by this snail species. In captivity, A. mustelina has been grown on cornstarch or sooty mold, with a supplement of cuttlebone providing the calcium necessary for shell growth. (Killian, 2007; Kobayashi and Hadfield, 1996)

Ecosystem Roles

Achatinella mustelina consumes fungi from plant bark and leaves. Historically, A. mustelina had no natural predators that relied upon it as a food source. With the introduction of the predatory snail Euglandina rosea, A. mustelina, along with other sympatric members of the genus Achetinella have become prey for this species. (Hadfield and Mountain, 1980)

Economic Importance for Humans: Positive

Due to their vibrant coloration, Achatinella mustelina shells were collected by human inhabitants of the island of Oahu to craft traditional leis and other ornaments. Shells from the genus Achatinella are still collected and sold as ornaments today as part of Hawaii’s tourist trade. (Hart, 1978)

Economic Importance for Humans: Negative

There are no known negative effects of Achatinella mustelina on humans.

Conservation Status

Achatinella mustelina is the currently the most abundant species of the O’ahu tree snails and has been studied considerably for conservation efforts. The slow growth, long pre-reproductive life, and low fertility of A. mustelina, in conjunction with its relatively sedentary lifestyle and small geographic range, make A. mustelina populations very vulnerable to disturbances, either from predation, human collection, or habitat destruction. Lowland habitat destruction by human inhabitants for the purposes of farming and logging have reduced the geographic range of A. mustelina to only high elevation mountainous forests.

However, the introduction of the invasive predatory snail Euglandina rosea from North America led to the rapid and widespread decline and destruction of A. mustelina populations. Euglandina rosea predates opportunistically on a number of species of terrestrial snails. Only a few hundred individuals of A. mustelina are estimated to remain in the wild. (Hadfield, 1986; Hadfield, et al., 1993)

Contributors

Peter Bicescu (author), The College of New Jersey, Colleen Stalter (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Renee Mulcrone (editor), Special Projects.

Glossary

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

island endemic

animals that live only on an island or set of islands.

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

mycophage

an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

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This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.